1. Field of the Invention
This invention relates generally to photovoltaic cells and a method for making them. More specifically, it relates to improvements in stability and lifetime of these cells. Such cells are useful for converting solar energy into electricity.
2. Background
Stability and degradation problems have long plagued Cu.sub.x S/CdS solar cells. These problems are intimately related, in part, to the stoichiometry of the copper sulfide layer. Stoichiometry, which is a phase sum of mainly two dominant phases (chalcocite and djurleite), is important as optimum efficiency is considered to be obtained when the copper sulfide layer is predominantly chalcocite (Cu.sub.x S; 1.992&lt;X&lt;1.998) phase. Efficiency is degraded as the copper sulfide is oxidized to a higher oxidation state of copper, for example, to djurleite (Cu.sub.1.95 S) or digenite (Cu.sub.1.7 S). Several intrinsic mechanisms are thought to contribute to the degradation of the desired chalcocite phase to other phases causing the cells to gradually reduce their electrical output with time. Oxidation by external media as well as by copper migration through the layer via dislocation, stacking faults, subgrain boundaries and similar defects are thought to contribute to the chalcocite phase degreadation. Light and moisture are the dominant culprits in the chalcocite phase decay. Elevated temperature can enhance degradation.
U.S. Pat. No. 3,888,697 issued June 10, 1975 to Bogus et al obtains an improvement in stoichiometry of the cuprous sulfide layer by depositing on the copper sulfide layer an additional layer of copper metal. Presumably, the excess copper metal reduces any higher oxidation state copper present to produce chalcocite without altering defect-dominated chalcocite decay kinetics called the "memory effects" in Cu.sub.x S/CdS. The application of an additional chromium layer as provided by the instant invention provides prolonged chalcocite stability and a change in the built-in memory of Cu.sub.X S/CdS cells.
Japanese Pat. No. 4,824,675 issued July 23, 1973, to Hamaski purports to stabilize the properties of a solar cell by preventing oxidation of the copper ion by an electric current of excessive amperage, as may occur when the cell is in the short circuited condition or under heavy load, by adding a small quantity of metallic iron to the copper sulfide. Presumably, the metallic iron acts similar to metallic copper in being oxidized in preference to the cuprous sulfide.
Metallization of the copper sulfide layer by chromium of the instant invention has a three-fold purpose,
(1) Chalcocite phase stabilization in the copper sulfide layer
(2) Throttlement of defect induced copper electromigration, and
(3) Formation of a coherent, passive oxide Cr.sub.2 O.sub.3 layer which protects relatively unstable chalcocite from ambient.